A major limitation to the maximum reach of ultra-long haul on-off transmission systems, such as 10 Gb/s WDM transmission systems, is pulse timing jitter. Pulse timing jitter arises from inter-channel pulse collisions, which develop as a result of the chromatic dispersion of a fiber, resulting in a wavelength dependent signal propagation velocity. Other sources of timing jitter can be attributed to contributions from transmitter and receiver electronics, acoustic interaction effects, or especially, in the case of soliton transmission, to the Gordon-Haus effect.
Inter-channel pulse collisions occur when pulses of shorter wavelength channels, which have higher velocities than those of longer wavelength channels, overtake and pass through pulses in the longer wavelength channels. Since real data streams are substantially random, some pulses tend to experience more collisions and others experience fewer collisions in the course of traversing the system. The time displacements of the collisions, therefore, can result in considerable timing jitter.
The Gordon-Haus effect is caused by the interaction of soliton pulses with amplifier spontaneous emission (ASE) noise present along the transmission medium. J. P. Gordon et al. describe this effect in “Random Walk of Coherently Amplified Solitons in Optical Fiber Transmission,” Optic Letters, 11(10), pp. 665-7 (1986). ASE noise randomly alters both the amplitude and carrier or channel frequency of propagating soliton pulses. The frequency shifts result in jitter in pulse arrival times. Pulse timing jitter can subsequently cause a soliton pulse to shift into the time interval reserved for a neighboring soliton pulse. The result, often known as intersymbol interference, culminates as an error in the received information.
The above described sources of timing jitter in optical signals propagating along a transmission system, as well as others, can create errors when attempting to receive the propagating optical signals. In conventional receivers, an optical pulse is sampled once within its bit slot. Because clock recovery circuits of conventional receivers cannot track fast timing jitter, the adjustment of an optimal pulse sampling point on every bit to attempt to minimize the receiving errors due to timing jitter is impossible. As such, timing jitter can cause an increase in the bit-error rate of an optical transmission system because of the shifting of the frequency of an optical pulse.